1. Field of the Invention
This invention relates to A/D converters using a dual slope integration circuit and, more particularly, to such A/D converter which is constructed so as to produce a digital signal proportional to the square root of the input signal.
2. Description of the Prior Art
For obtaining an A/D converted value in the compressed form, conventionally there have been practised two different ways; the analog value after having been compressed by an analog compression circuit is converted to a digital value, or the analog value is merely converted to a digital value which is then compressed by computation. In the former case, however, the circuit becomes complicated. In the latter case, it takes a long time and the dynamic range is narrowed, so that the required accuracy cannot be obtained.
On this account, in the case of using the dual-slope A/D converter, the inverse-integrated current generating means can be provided with control means for changing the inverse integration current in such a way that the inverse integrated charge amount increases exponentially as the inverse integration time increases. The dual-slope A/D converter having such input response output characteristic with the logarithmic compressing function is disclosed in FIG. 4 of U.S. Pat. No. 4,377,742.
In another case where the analog quantity is a square function of a given parameter, for example, the consumed electrical power varying as a square function of the supplied voltage is used and the supplied voltage is to be found digitally from the consumed electrical power, and the density of the photographic negative as a square function of the brightness is used and the brightness is to be found from the density of the negative, there have been two methods, one of which is that after the mere analog-to-digital conversion of the consumed electrical power or the density of the negative, the digital signal is compressed, and another method which is that the consumed electrical power or the density of the negative while being in the form of an analog signal, is compressed and the A/D conversion then follows. By the former method, it takes a long time to compress the digital signal, and, because the dynamic range of the consumed electrical power or the photographic density is very wide, it also takes another long time to perform the A/D conversion.
Even in the latter method, there is an alternate drawback in that the circuit for compressing the analog signal becomes complicated.